Variable Refrigerant Volume (VRF) System

Overview

This template generates a central DOAS air handling unit that provides a constant volume of conditioned outside air to all zones. It also generates multi-split DX heat-pump system with vrf zone units in each zone and the outdoor unit.


Application 

This template system only applies to vrf system units. 


Air Handling Unit (AHU) – Return Air

The table below describes the key properties of the AHU for this system where there is return air:

AHU Feature

Default

Extent of user-control

Airflow Rate

Constant area-weighted rate for all zones

Can change constant

Supply Air dry bulb Temperature

Constant with defaults as follows:

Can change constant value

Supply Air Temperature Reset Controls

None

No control

Supply Air Dew Point Controls

Constant with defaults as follows:

Min: -5 C or 23 F

Max: 13 C or 55.4 F

None

Supply Air Humidification Controls

Via Min Supply Air Dew Point Control

None

Peak Fan Power

3.5 W/(L/s) or

1.65 W/cfm

Can control peak load

Fan Curve

Standard ASHRAE.

Click to see fan curve

No control

Outside air configuration

100% outside air

No control

Return Air Rate

100% of air assumed to be returned for heat recovery

No control

Heat Recovery Efficiency

Sensible and Latent effectiveness available. Default is 50% sensible and 0% Latent.

User can change sensible and latent effectiveness

Cooling Coil

Direct Expansion coil post heat recovery

No control

Heating Coil

Hot water coil post chilled water coil

Can change to electric coil or direct gas

Frost Coil

Electric coil pre-heat recovery

Can change to hot water

Frost Coil Setpoint

Setpoint for coil – default is 2C

Can Change default


Outdoor VRF Unit

Outdoor VRF unit is self-contained condenser/evaporator (cooling/heating mode) that is connected to the multiple zone VRF units via refrigerant loop. System is designed to vary the amount of refrigerant flow in the loop based on the actual cooling/heating load.

System is capable of providing both cooling and heating both simultaneously and independently.

The table below describes the key properties of the zone level VRF units for this system:

Fan Coil Unit Feature

Default

Extent of user-control

Heat Rejection Type

Water-cooled

Can change to Air-cooled

Cooling Type

Direct Expansion

No control

Cooling Efficiency

COP with a default of 3.5

Control over COP

Cooling Efficiency Curve

Click to see curve

No control

Heating Efficiency

COP with a default of 3.5

Control over COP

Cooling Efficiency Curve

Click to see curve

No control


Zone VRF Units

 Zone VRF units are self-contained DX fan coil units that are connected to the outdoor unit(condenser/evaporator) via refrigerant loop. System is capable of varying the amount of refrigerant flow in the loop based on the actual cooling/heating load.

The zone VRF units are capable of providing both cooling and heating.

The table below describes the key properties of the zone level VRF units for this system:

Fan Coil Unit Feature

Default

Extent of user-control

Heating Coil Type

Heat pump

Can change to electric or gas coil

Cooling Type

Direct Expansion

No control

Fan Coil Motor Type

On/Off

No control

Peak Fan Power

0.64 W/(L/s) or

0.3 W/cfm

Can control peak load

Fan Curve

Constant Volume Fan

No control

Supply Air Temperature - Heating

Constant for all zones (35C / 95F)

User can change constant

Supply Air Temperature - Cooling

Constant for all zones (13C / 55.4F)

User can change constant

Demand Control Ventilation

None

None

Dehumidification Load

Air dehumidified to achieve the cooling setpoint at 60% Relative Humidity in the zone

None


Central Plant Configuration

The base case option is assumed to have a central plant that provides heating hot water to the AHU and zone.  

Central Plant Feature

Default

Extent of user-control

Heating Hot Water Source

Natural Gas (Water-cooled system only)

Can change to electric boiler

Boiler Peak Efficiency

Assumed to be 0.9

Value can be changed

Boiler Performance Curve

Click to see curve

No Control

Supply and Return Temperatures – hot water

80/70 C or

176/158 F

User can control

System losses

Constant loss of 15%

User can control loss rate only.

Peak Pumping Energy

1.32 W/(L/s) or

21 W/gpm

No control

Pump Efficiency Controls

Variable Speed Drives

No control

Efficiency Curve for Variable Pumping

Click to see pump curve

No control

Pumping Configuration

Primary Only

No control

Condenser Water Source (Water-cooled system only)

Cooling Tower

Ground Heat Exchanger

Cooling Tower Efficiency

245.7 W/(L/s) or

15.5 W/gpm

Value can be changed

Supply and Return Temperatures – condenser water

29/35 C or

84.2/95 F

User can control

System losses

Constant loss at 5%

User can control loss rate only.

Was this article helpful?
0 out of 1 found this helpful

Comments